Opioids are effective analgesics, however, their use is associated with a number of undesirable side effects. One of these side effects is pruritus, or itching. Pruritus is a common side effect associated with the use of opioids and may be very severe. Pruritus can occur when the opioid is administered intramuscularly, intravenously, transdermally, transmucosally or intrathecally.
It is believed that the opioid induced pruritus results from the release of histamine in response to the administration of opioids. Opioids are thought to stimulate histamine release by binding to opioid receptors on the central nervous system. This, in turn, causes peripheral nerves and histamine containing cells to release histamine.
Based on this theory a number of treatments have been used to alleviate opioid induced pruritus. The first is the use of antihistamines. However, antihistamines have a variable effect on opioid induced pruritus. Additionally, the use of antihistamines, when effective, only treats the symptom after it has occurred, rather than preventing its occurrence.
Another undesirable side effect of opioids is urinary retention, or the patient's inability to spontaneously empty his or her bladder. This urinary retention is a common side effect that can occur when opioids or related compounds are administered intramuscularly, intravenously, transmucosally, transdermally, or intrathecally. It is not clear why opioids cause urinary retention, but it is thought to be related to the central anticholinergic stimulation that opioids induce. Based on this theory, a number of cholinergic-type drugs have been used to treat urinary retention. However, due to the side effects of cholinergic drugs, catheterization of the bladder with a tube to drain urine remains the mainstay of treatment.
Another opioid-induced side effect is dysphoria, a feeling of unpleasantness or discomfort. Many subjects, especially those without pain, report unpleasant psychomimetic responses to the administration of an opioid alone. These responses have been previously attributed to activation of centrally located opioid receptors. This opioid-induced dysphoria is commonly treated by the addition of other drugs, such as benzodiazepines, to decrease the dysphoria or to blunt the recall of the dysphoria. These drugs, however are associated with increased levels of sedation and may enhance respiratory depression caused by the opioid.
One treatment for side effects such as pruritis, urinary retention and dysphoria is the use of opioid antagonists which cross the blood-brain-barrier, or which are administered directly into the central nervous system. Opioid antagonists such as naltrexone and naloxone have been administered intramuscularly or orally to treat opioid induced pruritus. Naltrexone and naloxone are highly lipid soluble and rapidly diffuse across biological membranes, including the blood-brain-barrier. However, naltrexone, naloxone and other opioid antagonists also reduce the analgesic effect of the opioid being used.
Many quaternary amine opioid antagonist derivatives, such as methylnaltrexone, do not reduce the analgesic effect of the opioids. These quaternary amine opioid antagonist derivatives, which have a relatively higher polarity and reduced lipid solubility when compared to the tertiary forms of the drugs, were specifically developed to not traverse the blood-brain-barrier or to traverse it at a greatly reduced rate. Since these quaternary opioid antagonist derivatives do not cross the blood-brain-barrier, peripheral administration of these antagonists would not be expected to be effective in the treatment of an opioid induced side effect caused by the opioid within the central nervous system. In fact, experiments show that to be effective in blocking the opioid receptors in the central nervous system, these antagonists must be injected directly into the central nervous system. However, injection of drugs directly into the central nervous system is undesirable since it increases the possibility of introducing bacterial or viral contamination to the central nervous system.
It is desirable in the treatment of many conditions to have oral medications with prolonged effects. Such oral medications are particularly desirable both for the treatment of opioid-induced side effects (such as urinary retention, pruritus, and some forms of constipation) and for the treatment of nonopioid-induced side effects (such as other forms of constipation and delayed gastric emptying from enteric feeding).
It is further desirable to develop a method for the prevention of opioid induced dysphoria, opioid induced pruritus, urinary retention, opioid- or nonopioid-induced delayed gastric emptying from enteric feeding, and constipation, which does not counteract the analgesic effects of the opioid, or risk increased levels of pain.